Ultrasound imaging, or ultrasonography, is a convenient imaging modality for diagnostic purposes. It may be employed before, during or after a therapeutic intervention. Typically, a healthcare professional uses a hand-held probe, or transducer, which is moved as appropriate to visualize reference structures. In many cases, the transducer is placed on the surface of the body. However, in some specialized procedures, such as endovaginal, endorectal and transesophageal procedures, specific transducers are placed inside the patient's body. Small transducers may even be mounted on catheters and inserted into blood vessels to image the vessel walls.
Recent advances in ultrasound require real-time monitoring in an increasing range of diagnostic and interventional procedures. For example, for the diagnosis and treatment of an atrial septal defect (ASD) or a patent foramen ovale (PFO), the healthcare professional may employ:                Transesophageal Echocardiography (TEE), where the ultrasound transducer is placed inside the esophagus        Transthoracic Echocardiography (TTE), where the ultrasound transducer is placed on the chest, outside the body.        Intracardiac Echo (ICE), where the ultrasound transducer is placed in the venous system and advanced to the heart.        
During procedures requiring visualization, the user coordinates the movement of the transducer by the manual selection of an appropriate representation on the screen, such as a 2-D viewing cross-section of the 3-D imaging volume. In systems of the prior art, such as disclosed in U.S. Pat. No. 6,342,889, a system is provided that provides an initial view, the user then selects points of interest, and the system provides a representation including the selected points on a display. Although a degree of automation is provided, the operator of the system is required to frequently input the points of interest. While this may be acceptable in off-line applications where the user is processing previously acquired imaging data, it is not acceptable in real-time applications, where the input of the user directly affects the accuracy and reliability of the procedure being performed. Users will be required to make choices and make viewing selections throughout the procedure, and even highly trained operators will often be required to perform trial-and-error to obtain the desired results.
Ultrasound may also be employed for functional measurements, such as Doppler measurements, where the Doppler effect is exploited to measure the direction and speed of movement of a reference structure, for example a portion of a heart valve or a jet of blood flow in a vessel, relative to the transducer. Typically, the measurement requires the manual selection of target planes, lines, or volumes where the measurement is to be performed.
Doppler measurements may be performed using both continuous and pulsed systems, with pulsed systems having the advantage that distance information about the depth or range of the reference structure may be obtained from the ultrasound pulses.
However, pulsed Doppler is known to suffer from aliasing if the velocity of the reference structure and the angle between the measurement beam and the blood flow direction combine to give a Doppler frequency greater than half of the pulse repetition frequency. This creates ambiguity in the Doppler signal, and may cause misinterpretation of the reference object's movement. Typically low velocities, for example venous flow, are measured using low pulse repetition frequencies, and high velocities, for example arterial flow, are measured using higher pulse repetition frequencies.
An additional problem of pulsed Doppler is that the depth of measurement is limited by the pulse repetition frequency chosen, because the time interval must be sufficient to allow a pulse to travel from the transducer to the reference structure and back, before the next pulse is emitted.
Systems are known in the prior art which combine the visualization and functional possibilities. This may be done using different transducers, or more commonly a single transducer which can operate in two different modes, for example pulsed Doppler for the functional mode and B-mode imaging for the visualization.
In Color Doppler ultrasound, the Doppler shifts in a few thousand sample volumes in an image plane are measured. For each sample volume, the average Doppler shift is encoded as a color, and displayed on top of the B-mode image. Again the transducer is switched between two different modes of operation.
The measurement of samples and the processing of the results require considerable computational power, making such an instrument expensive.
Furthermore, functional measurements in general require the ultrasound device to be configured appropriately in terms of transducer orientation, selection of measurement volume, selection of the pulse repetition frequency etc., so that the ultrasound beam is reflected precisely at the selected reference structure. The position and extent of the measurement volume is conventionally selected by the user, and therefore can result in inaccuracies in the results measured.
In some cases, ultrasound may be employed during an intervention using a surgical instrument, for example a catheter in the treatment of an atrial septal defect (ASD) or a patent foramen ovale (PFO).
PCT application WO 2005/101277 discloses a system that provides in real-time three-dimensional imaging for use during an intervention with a biopsy needle. This system segments the biopsy needle from the volume, using a Hough Transform to give the position and elongation of the needle. This may then be used to automatically select image slices, such that the user always looks in the direction of the biopsy needle.
Although acceptable for some applications, this system can only provide a limited field of view from the viewpoint of the biopsy needle, making it very easy for the user to navigate the needle incorrectly and to lose orientation. The user must then manually select image planes to regain the orientation, or even move the needle back until orientation is restored.
Therapeutic applications of ultrasound are also known in the art. They provide localized heating and/or mechanical agitation in anatomical structures. For example, Focused Ultrasound Surgery (FUS) or High-Intensity Focused Ultrasound (HIFU) may be used to heat-up reference structures such as cysts and tumors. In another example, lithotripsy employs ultrasound to break up reference structures such as stones in the kidney, bladder, ureter or gall bladder. Such applications typically employ higher energies than for visualization or functional measurement, and therefore inaccuracies in positioning may result in damage to surrounding tissues. In the art it is known for the user to determine the position using a modality suitable for visualization, such as MRI.